The present invention concerns a start-off control method and system for a motor vehicle using dialogue between the engine control computer and the electronically controlled mechanical gearbox when restarting the vehicle in stop-and-start mode.
A so-called “stop-and-start” system is characterized in that the engine is turned off just before and while the vehicle is at a standstill (at traffic lights, in traffic jams, etc.). It automatically and instantaneously turns back on when the brake pedal is released when the driver wants to proceed.
A stop-and-start system offers clear advantages during engine standby phases: auditory comfort for the passengers, physical comfort due to eliminating the engine vibrations, and driving comfort, not to mention an improvement in fuel consumption.
An engine must start and transmit torque as quickly as possible. Actually, the vehicle acceptability criterion for starting off a vehicle on an uphill slope is that it move backwards no more than 20 cm on a 10% slope. This requirement effectively means that one must have an available torque of 80 N·m (Newton·meters) on the engine shaft 400 ms (milliseconds) after the brake is released.
The operating point that allows the electronically controlled mechanical gearbox (ECMG) to begin the actual “start-off” (or forward movement) of the vehicle is signaled by the engine control computer (ECC) to the electronically controlled mechanical gearbox (ECMG).
The ECMG is a gearbox with electronic control of the clutch and gear shifting. It can shift gears automatically or sequentially on the driver's initiative.
The signal from the ECC to the ECMG is sent via the CAN bus in the form of a flag, known as the “ECMG start-off authorization”. A flag is a variable whose value is set at 0 or 1 based on an event; then the value of the flag is tested to find out whether or not the marked event has occurred. The CAN bus (Controller Area Network) is a network communication protocol for linking the on-vehicle equipment of a motor vehicle together.